[DeathsHand]

I learned something along the lines of by 4-5 months of age, babies realize basic principles of how the world works; things fall towards the ground if nothing is supporting them, things don't just "disappear" when they go out of view, etc.
And if you try to confuse them with 'magic' tricks, they'll get very confused and upset.
If they are even a few weeks older, they are actually able to rationalize what you're showing them, and get bored with the 'trick'.
But that if you anticipate their rationalization, and find a way to 'disprove' it (even though their assumptions that you're tricking them are correct), they will once again get very upset and confused.
"That's not what's supposed to happen!", they will think. But be unable to say.

Or so Psycho 201, with the help of Alan Alda, tells me...
It was all very cute and interesting.